Not applicable.
Not applicable.
The invention relates to a method for the nondestructive measurement of the thickness of thin layers with a probe for nondestructive measurement of the thickness of thin layers and to an apparatus for carrying out the method according to the invention.
Curved and coated surfaces can be encountered in particular in aircraft construction, in automobiles, in moulded parts, in the area of household articles, in pipeline construction, on Venetian blinds and in the case of many other coated objects of measurement. In the area of nuclear power as well, oxide layers on zirconium heat exchanger tubes, in the range of 20 xcexcm on tubes approximately 12 mm in diameter, are to be measured.
Known tactile measuring methods, which are based for example on the eddy current principle, are influenced very strongly by the shape of the object of measurement. For this reason, when there are different curvatures a calibration must take place, which is time-consuming and may lead to errors, in particular because changes in curvature are not taken into account.
German Patent Document DE 41 19 903 A1 discloses a method and an apparatus for the measurement of thin layers, making it possible that the undesired dependence of the measured value on the geometrical shape of the object of measurement can be eliminated in a broad range. This means that it is made possible for thin layers to be measured even on curved surfaces of objects of measurement. These are layer thicknesses from just a few xcexcm to several 100 mm. This nondestructive layer thickness measurement relies on the eddy current principle, which is based on changes of a low frequency or high-frequency electromagnetic field in dependence on a layer applied to the object of measurement. Provided for this is an apparatus which has a first coil device on a ferrite core. An outer sleeve, which surrounds the first coil device, receives a second coil device, lying outside. The core receiving the first coil device has at its lower end a hemispherical placement dome of abrasion-resistant material.
To ascertain the layer, the probe is placed on the object of measurement. The two coil devices are excited with frequencies which are different at the same time and consequently emit two signals of different frequencies during the measurement, which are evaluated by a suitable circuit in order to calculate the layer thickness. The layer thickness is ascertained in accordance with the equations specified in DE 41 19 903 A1.
The invention is thus based on the object of improving the quality of the ascertained layer thickness values in the nondestructive measurement of the thickness of thin layers.
This object is achieved by the steps of using a probe having a first coil device on an inner core, the geometrical center of which coil device and the geometrical of at least a second coil device coincide, the at least second coil device partially surrounding the first coil device, using an evaluation unit, to which signals of the coil devices are emitted during a measurement for ascertaining the layer thickness, using a circuit, by which the first and the at least second coil device are excited sequentially during a measurement. The sequential excitation of at least two coil devices allows nondestructive emission of the measuring signals to take place from the coil devices to an evaluation unit. The successive excitation of the coil devices with the respective frequency allows a signal uninfluenced by the respectively neighbouring other coil device to be recorded by the evaluation unit during the measurement and also unequivocally assigned to each coil device. As a result, overshooting or sympathetic excitation of the neighbouring coil device can be eliminated, whereby the signal emission can take place free from disturbing parameters.
According to a further refinement of the method, it is provided that the coils are excited with high frequency. As a result, changes of the alternating electromagnetic field when a probe head approaches the object of measurement can be utilised as a measuring effect for the measurement. When high-frequency fields are used, the layers to be measured are electrically non-conducting, such as paint for example, or weakly conducting, such as chromium or the like for example.
According to a further refinement of the method, it is provided that the frequency signals coming from the first and at least second coil device, which are emitted at separate times from one another, are restricted in the duration of the emission of the signals by field-effect transistors. As a result, a separation between the emitted frequency signal of the first coil device and of the at least second coil device can be ensured, whereby an exact recording and assignment of the measuring signals can be achieved. As a result, mutual influencing or superimposing can be prevented, even during the transmission of the signals to the evaluation unit. The activation of the coil devices and of the field-effect transistors, via which the measuring signals are passed on to an evaluation unit, can take place in a defined time window, whereby the assigning of the measuring signals is also made possible.
According to a further refinement of the method, it is provided that the signals emitted by the coil devices are evaluated independently of one another. For example, a signal substantially determining the layer thickness may be recorded by a first coil device, while for example a signal determining the curvature of the object of measurement may be recorded through the other coil device. The separate evaluation allows an exact calculation of the two measurement variables to be obtained, whereby the subsequent ascertainment of the measured value can take place by the formulaexe2x80x94as revealed by DE 41 19 903 A1xe2x80x94with a higher degree of accuracy.
The separation of the signals of the coil devices advantageously allows the excitation of the coil devices to take place with the same frequency, whereby further simplification of the structural set-up of the control system can be achieved.
According to a further refinement of the method, it is provided that, for carrying out a measurement, a first coil device is excited by a first circuit and a second coil device is excited in a second circuit one after the other by means of a flip-flop circuit. The provided field-effect transistors make it possible to ensure by the circuitry used that the sequential activation of the coil device is made possible for the emission of frequency signals. It is provided that the circuits of the coil devices are identically designed. As a result, the set-up of the circuit can be of a simple arrangement.
According to a further refinement of the method, it is provided that the frequency signals are passed to the evaluation unit via a compensator. This makes it possible for the phase relationship of the frequency signals to be set in such a way that the undesired dependence of the measuring signals on the electrical conductivity of the base material is largely eliminated.
Consequently, the method according to the invention for the nondestructive measurement of the thickness of thin layers allows the effect to be achieved that the influence on the measured values at least of disturbing parameters such as the curvature of the surfaces and the conductivity of the object of measurement, which have particularly critical effects in the measurement of thin layers in the range of just a few xcexcm, is eliminated virtually completely.
The apparatus according to the invention, which is intended in particular for carrying out the method, has in a housing a ferritic cup-type core, which receives a first coil device and has on a pin lying in the first coil device a hemispherical placement dome and receives a further coil device, outside the cup-shaped core, concentrically with respect to the first coil device. The coil devices, which are arranged fixedly in relation to each other and have a common centre axis, allow an exact ascertainment to be obtained, since the range of action of the first and second coil devices emanates from the common geometrical axis on account of the common placement point of the hemispherical placement dome. The first or inner coil device is in this case designed to be sensitive to layer thicknesses and the outer or second coil device is designed to be sensitive to curvature.